1. Field of the Invention
The present invention relates to a metallized polyimide film that includes a polyimide film and a conductive layer made of copper or the like formed on the surface of the polyimide film, and can be used as, for example, a TAB tape, a flexible circuit board, or a flexible wiring board.
Priority is claimed on Japanese Patent Application No. 2004-27661, filed Feb. 4, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, in order to achieve reduction in the size and weight of electronic apparatuses and enhancement of the flexibility of the structure, there has been a growing demand for tape automated bonding (TAB) tapes, flexible circuit boards, flexible wiring boards, and the like. As substrates of these boards, flexible plastic substrates having a copper foil bonded thereon using adhesives, such as epoxy adhesive, have been employed conventionally.
However, the demand for high-density packaging of electronic apparatuses has required further reduction in the thickness of these substrates. However, the above-described flexible plastic substrates that are bonded with copper foil cannot meet this requirement for reducing the thickness.
In addition, the above-described circuit board using adhesive has several shortcomings as follows:    (1) An etching solution that is used in an etching of a copper foil easily migrates into an adhesive layer. When bias is applied under a high temperature and high humidity condition, copper may migrate to a plastic substrate, resulting in short-circuiting.    (2) For enhancing the operating speed, both impedance matching and reduction in crosstalk are required, both of which are difficult to achieve when an adhesive is used.    (3) The adhesive layer has poor dimensional stability.    (4) It is difficult to achieve high-density packaging since small patterns cannot be formed on a circuit board having the adhesive layer.    (5) The adhesive layer exhibits poor heat stability since the adhesive has inferior thermal properties compared to those of the material of a plastic substrate, which makes high-density packaging difficult.    (6) The circuit board easily deforms due to the adhesive.
In order to solve these problems, new techniques for forming a metallized film without using an adhesive have been studied. Several such techniques are known. For example, in one method, a metal thin film is formed directly on a plastic film according to circuit patterns using thin film formation techniques, including vacuum evaporation, sputtering, ion plating, or the like. Then, a metal plating layer is formed on the metal thin film using electroplating or the like. In another method, a metal thin film is formed on the surface of a plastic substrate, and then a metal is deposited using electroplating or the like. The conductive layer is etched to define circuit patterns.
However, these methods also have shortcomings. In a polyimide film having an excellent heat resistance, the bonding strength between the polyimide film and a metal is weaker than the bonding strength between the metal and other plastics. Thus, the bonding strength between the polyimide film and the metal thin film is reduced after the circuit pattern formation or the electroplating process is carried out, which may often result in peeling off.
Other methods have been used; for example, in one method, a polyimide monomer is coated on a copper foil and then heat cured to form a polyimide layer. In another method, a thermoplastic polyimide film and a copper foil are bonded together using the heat seal technique.
However, these methods have shortcomings in that the surface roughness of a metallized polyimide film is affected by the surface roughness of the copper foil employed, which may adversely affect a fine pattern formation process. In addition, since there is a limit to reducing the thickness of copper foils, it is difficult to achieve reduction in thickness.
In order to address these shortcomings, the present inventor proposed a metallized polyimide film including a polyimide film, a metal nucleus adhesion region formed on the surface of the polyimide film, and a metal layer formed on the surface of the metal nucleus adhesion region, which are disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-011273.
The metal nucleus adhesion region is made of at least one metal selected from the group consisting of Mo, Cr, Ni, Si, Fe, and Al.
As applications of metallized polyimide films has become widespread, the metallized polyimide films are used in further harsh environments, which has created the demands for metallized polyimide films that have an excellent durability and extended service life at high temperatures and high humidity.
The present inventor has studied a metallized polyimide film that can be used at a high temperature environment, and it was discovered that the following events can occur when a metallized polyimide film is exposed to a temperature higher than the temperature at which a metallized polyimide film is used conventionally in the presence of oxygen and water.
When the metallized polyimide film is exposed to a high temperature, oxygen and water gradually infiltrate from the backside of the polyimide film and permeate into the polyimide film. If the oxygen and water infiltrate to the copper layer, the copper is oxidized. As a result, the decomposition of the polyimide due to hydrolysis reaction between the resulting copper ions and the polyimide occurs. Thus, the polyimide film is gradually decomposed in the vicinity of the interface with the copper layer, and the bonding strength between the copper layer and the polyimide film may weaken over a long period of time.